Your first problem probably will be that the 28mm and the 32mm are different in intake spigot size. After that the fun begins. Try finding a RM250 or PE 250 with a 32mm stock carb specs and then replicate those. For the inexperienced, and feint of heart this can be a daunting task and a large expenditure............................in other words a lot of money spent that gets you no where. Sorry for the bad news warning. I've been there and done that, and know it takes lots of experience, skill, patience, and understanding of the carb circuits to get the job done.
OD